Gypsum products can be manufactured using a slurry formed from at least water and stucco. The stucco, which is calcium sulfate hemihydrate (CaSO4.½H2O), reacts with water to form gypsum, which is calcium sulfate dihydrate (CaSO4.2H2O). The two water molecules in crystallized gypsum are chemically bound to the calcium sulfate in what is often termed “crystallized water.”
Gypsum wallboard is generally a composite board comprising a core 104, face sheet/liner 106, and back sheet/liner 108 (FIG. 1). In a wallboard assembly, the face sheet 114 of the wallboard is exposed to the exterior, and the back sheet 116 is placed inside a cavity defined by two wallboards (FIG. 2). Gypsum wallboards are commonly used in drywall construction of interior walls and ceilings, and should be able to withstand both fire and excessive temperatures. As a result, gypsum wallboards are manufactured using specifications that maximize fire endurance/resistance.
Fire endurance/resistance of gypsum wallboard is measured by the period for which a board can withstand a standard fire test. The fire resistance of a wallboard is classified according to the ability for a wallboard to avoid an increase in temperature, flame passage, and structural collapse. In order to have various parties, including constructors, occupants, and regulating bodies, evaluate the fire endurance on a common basis, fire test assemblies are categorized into several standard arrangements. Some common assemblies include test designs defined by Underwriters Laboratories, Inc. (UL®), a testing and certification agency, which has tests that are referred to as U305, U419, and U423.
A standard fire test is customarily conducted in accordance with the requirements of ASTM E119 (2009). In such tests, a fire resistance classification can be established based on the time at which a wall assembly shows excessive temperature rise, or passage of flame, or structural collapse. Failure of the test occurs when the average temperature as measured by several thermocouples on the unexposed surface increases more than 250° F. above ambient temperature, or any individual thermocouple rises more than 325° F. above ambient temperature. The duration of fire endurance of a system is not only dependent upon the gypsum board used in the system, but also depends upon many other factors, including wall assembly thickness, stud type and spacing, board size, insulation type, and others.
Although existing techniques are useful in extending wallboard fire endurance and resistance, further improvement is always desirable.